1. Field of the Invention
The present invention relates to an engine equipped with a multi-link (double-link or other multi-link) type piston crank mechanism, and, more particularly, to an engine with a multi-link type piston crank mechanism constructed in such a way that a piston stays in the vicinity of the top dead center longer than a conventional piston. The present invention can be applied to an engine equipped with a multi-link type piston crank mechanism and is effective in improving the heat efficiency of the engine to reduce fuel consumption.
2. Description of Related Art
Japanese Laid-Open Patent Publication No. 2003-232233 (related to U.S. Pat. No. 6,792,924, which is incorporated by reference in its entirety herein) discloses an engine which has a double-link type piston crank mechanism, and increases a compression ratio of the engine to improve fuel consumption, by raising the top dead center position of a piston in a partial load operation mode using the characteristics of the double-link type piston crank mechanism.
When the compression ratio is increased, the heat efficiency increases while the cooling loss caused by a rise in combustion temperature also increases. Accordingly, the engine disclosed in Japanese Laid-Open Patent Publication No. 2003-232233 drops the combustion temperature by executing sufficient exhaust gas recirculation, thereby suppressing the cooling loss. As the amount of the exhaust gas recirculation increases, the combustion speed gets slower, so that when combustion is carried out with a delay at a timing shifted from the top dead center (e.g., retarded ignition), the combustion becomes unstable. In this respect, the engine in Japanese Laid-Open Patent Publication No. 2003-232233 further uses its double-link type piston crank mechanism to change the piston motion in such a way that the piston stays longer in the vicinity of the top dead center, thereby preventing instability of the combustion.
Therefore, the engine in Japanese Laid-Open Patent Publication No. 2003-232233 can improve the heat efficiency by increasing the compression ratio; can reduce the cooling loss and pumping loss with sufficient exhaust gas recirculation (thus improving the fuel consumption); and can reduce the exhaust amount of nitrogen oxide(s) by decreasing the combustion temperature using exhaust gas recirculation.
However, the use of exhaust gas recirculation is not advantageous in all ways. For example, because exhaust gas recirculation dilutes the air/fuel mixture by recirculating the exhaust gases into the air/fuel charge, the heat efficiency achievable by using exhaust gas recirculation in the double-link crank mechanism is not optimum.